This invention relates to a novel construction for a ring laser angular rate sensor and more particularly to the anode-cathode electrodes for generating the laser beams of the sensor.
After years of development, ring laser angular rate sensors, commonly referred to as ring laser gyros, have become commercially successful products and are rapidly replacing conventional mechanical angular rate sensors in many applications. Most commercial ring laser angular rate sensors use a mechanically and thermally stable block construction and mechanical dither concepts taught in U.S. Pat. No. 3,390,606, issued to Podgorski, U.S. Pat. Nos. 3,467,472 and 3,373,650, issued to Killpatrick, which are all assigned to the assignee of the present invention. The above-referred to patents are incorporated herein by reference thereto.
Ring laser angular rate sensors, of the type referred to, commonly utilize a block of material that is dimensionally stable, both thermally and mechanically. The block usually includes a plurality of interconnected gas-containing tunnels or passages which form a closed-loop path in the shape of a triangle, a rectangle, or any polygonal path. At each intersection of a pair of interconnecting tunnels is a reflective surface provided by a mirror mounted on the block. This arrangement of mirrors and interconnected tunnels form an optical closed-loop path. Further, at least one anode and one cathode are each mounted on the block in communication with the gas. Each of the components, including the mirrors, anode, and cathode must be sealed to the block to form a gas tight seal. The block is usually filled with a lasing gas such as a mixture of helium and neon. If a sufficiently large electric potential is applied between the anode and cathode, a discharge current will flow therebetween and will result in a production of a pair of counter-propagating laser beams within the block.
Ring laser angular rate sensors, and specifically ring lasers, known in the art, utilize a anode-cathode combination which is in communication with the gas containing closed loop cavity. A sufficiently large direct current electric potential is applied between the anode and cathode to generate a direct electrical current between the anode and cathode through the gas. The electrical current through the gas creates distinct gas discharge regions of ionized gas atoms which are dependent on the cathode-anode configuration. In ring lasers of the prior art there usually exists (i) a cathode fall discharge region nearest the cathode surface, (ii) a negative glow discharge region in the hollow cathode, (iii) a positive column discharge in portions of the hollow cathode cavity, portions of the closed-loop optical cavity defined by the tunnels, and in proximity of the anode, and (iv).an anode fall discharge region in close proximity to the anode surface.
As is well understood by those skilled in the art, the positive column discharge will contain a population of excited gas atoms (population inversion) which will emit photons and begin the process of creating counter-propagating laser beams along the optical closed-loop path. The laser beams, once established, continually pass through the positive column discharge and generally collinear with the direction of current flow of the gas discharge current within the positive column.
The positive column discharge in the closed-loop cavity of a ring laser gives rise to gas circulation within the optical closed-loop cavity. Particularly, the positive column discharge in ring laser angular rate sensors of the prior art gives rise to gas circulation collinear with the counter-propagating laser beams of the ring laser and collinear with the discharge current direction within the positive column. This is thought to be attributed to momentum exchange between charged particles and the walls and charged particles and neutral particles, sometimes referred to as the Langmuir gas flow. In positive column discharge ring laser angular rate sensors of the prior art, this momentum exchange gives rise to moving gain atoms in the gas collinear with the laser beams. This results in large bias effects in the performance of the ring laser angular rate sensor. This bias appears as a difference in path length in the absence of rotation. This results in a false or biased sensor readout signal which results in an erroneous rotation rate indication or bias. Further, the positive column discharge ring laser angular rate sensors also gives rise to temperature gradients within the laser block which also impacts the bias and bias stability of the sensor.
In order to minimize the effects on the performance of the ring laser angular rate sensor due to positive column discharge operation, a symmetrical split DC discharge circuit has been utilized to provide "bias-balancing" and improve bias stability. This technique comprises, commonly, either a single cathode and a pair of anodes, or a single anode and a pair of cathodes symmetrically placed along the optical path length traversed by the laser beams. Two positive column discharges are created. The two discharges have opposite polarity as seen by the counter-propagating laser beams. In triangular ring lasers known in the art, the positive column created by the anode-cathode combination selected, results in a positive column in one direction, passing one of the laser mirrors, and results in a second positive column in the opposite direction, passing another of the laser mirrors. This unfortunately exposes the mirrors to the ionized gas in the form of the positive column discharge which can degrade the mirror performance.
A DC positive column discharge technique for ring lasers, specifically for ring laser angular rate sensors, have other disadvantages including, among others, stability of the gas discharge since a positive column exhibits a negative impedance which increases the tendency of the discharge to oscillate.